Simply remove tidb and deps (#3993)

models/models_tidb.go

@@ -1,18 +0,0 @@
-// +build tidb
-
-// Copyright 2015 The Gogs Authors. All rights reserved.
-// Use of this source code is governed by a MIT-style
-// license that can be found in the LICENSE file.
-
-package models
-
-import (
-	_ "github.com/go-xorm/tidb"
-	"github.com/ngaut/log"
-	_ "github.com/pingcap/tidb"
-)
-
-func init() {
-	EnableTiDB = true
-	log.SetLevelByString("error")
-}